Transparent touch screens produce electrical signals which correspond to a position on the touch screen being touched (activated) and are widely used as user input devices for electronic transaction terminal devices such as automated teller machines (ATMs) and point of sale (POS) terminals.
A resistive touch screen is probably the most widely used and cost effective touch screen on the market today. A resistive touch screen comprises a scratch-resistant layer that protects two conductive layers held apart by spacers. The scratch-resistant layer and the conductive layers are transparent to allow viewing of an underlying display such as display 100 in FIG. 1a. In a simplified model of a resistive touch screen, each conductive layer is modeled as a resistive component 102. The resistive component may be considered as having Y axis and X-axis resistances, shown for display 100 varying linearly from 0 to 100K ohms. An electrical current is conducted by the resistive touch screen when a voltage is applied across a conductive metallic layer. In general, one conductive layer of the resistive touch screen is operated such that current flows in the X-direction while the other conductive layer is operated such that current flows in the Y-direction. When an object touches the screen (“presses a pad”), for example at a position 104 (see FIG. 1b), the two conductive layers make contact. This creates a bridge resistance between the X and Y axis, dividing the X and Y axis resistances into RX1, RX2 and RY1, RY2 respectively.
An approach for determining the point of contact on the resistive touch screen, shown in FIG. 2a and FIG. 2b, is to separately sense a voltage at the point of contact in the x-direction (FIG. 2b) and the y-direction (FIG. 2a)—respectively Vx and Vy. Since each conductive layer is a resistor divider, sensing the voltage at the point of contact allows the position to be calculated. Knowing the position on the resistive touch screen in both the X and Y directions identifies the location of contact. In FIG. 2a the Vy measurement is done as shown, and the Vx measurement is done as shown in FIG. 2b. The touch screen is connected between a voltage Vcc and ground, with the voltages (“touch signals”) Vx, Vy sampled through an analog to digital (A/D) converter 206 by a processor (controller) 208 when a touch point (pad) 204 is pressed.
The use of touch screens in ATMs, POS devices, and other such devices, normally involves the entry of a personal identification number (PIN) as a form of identification. Any access to this PIN by an unauthorized party could be extremely damaging to the legitimate user's account. Such access may be achieved for example by eavesdropping on the electrical signals from a touch screen during PIN entry. These signals can then be used by the unauthorized party during a subsequent transaction to falsely identify itself as the PIN assignee. Eavesdropping may be done by monitoring the wires coupling the resistive touch screen to a main printed circuit board (system circuitry). The change in voltage on these wires could be detected easily, for example by parallel connection of a voltmeter to the wires or by inductively sensing the change in voltage. Known solutions to such eavesdropping include for example the solution suggested in U.S. Pat. No. 6,411,284. A reference signal input to the touch screen is varied randomly (in a way known only to the processor) such that voltages Vx, Vy change randomly for the same touch point along a time axis (from one touch “event” to the next). The reference signal may be based on voltage (Vcc), current or phase. Because the reference signal is analog, a digital to analog (D/A) converter is needed to provide digital inputs. By varying the reference signal, information entered on the touch screen is difficult to reproduce in a meaningful way by a third party eavesdropping on the x-axis signal and the y-axis signal.
Disadvantages of known solutions include requirements for D/A converters, which increase cost, as well as noise that is inherent in schemes based on varying reference signals. Another problem stems from the fact that it is rather intricate to calibrate the touch screen properly. It would therefore be advantageous to have inexpensive and simple to implement methods and systems which enhance the security of touch screens and which do not suffer from the disadvantages mentioned.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will become apparent after reading the following detailed description and reviewing the accompanying drawings.